In−situ observation for RH−dependent mixing states of submicron particles containing organic surfactants and inorganic salts

Abstract. Aerosol mixing state plays an important role in heterogeneous reactions and CCN activity. Organic surfactants could affect aerosol mixing state through bulk−surface partitioning. However, the mixing state of surfactant containing particles remains unclear due to the lack of direct measurements. Here, in−situ characterizations of mixing state for 20 kinds of submicron particles containing inorganic salts (NaCl and (NH₄)₂SO₄) and atmospheric organic surfactants (organosulfates, organosulfonates, and dicarboxylic acids) were conducted upon relative humidity (RH) cycling by Environmental Scanning Electron Microscopy (ESEM). As RH increased, surfactant shells inhibited water diffusion exposing to inorganic core, leading to notably increased inorganic deliquescence RH (88.3−99.5 %) compared with pure inorganic aerosol. Meanwhile, we directly observed obvious Ostwald ripening process, that is, the growth of larger crystals at the expense of smaller ones, in 6 among 10 NaCl−surfactants systems. As a result of water inhibition by surfactant shell, Ostwald ripening in all systems occurred at RH above 90 %, which were higher than reported RH range for pure NaCl measured at 27 ℃ (75−77 %). As RH decreased, 8 systems underwent liquid−liquid phase separation (LLPS) before efflorescence, showing a strong dependence on organic molecular oxygen−to−carbon ratio (O : C). Quantitatively, LLPS was always observed when O : C ≤ 0.4 and was never observed when O : C > ~0.57. Separation RH (SRH) of inorganic salt−organic surfactant mixtures generally followed the trend of (NH₄)₂SO₄ < NaCl, which is consistent with their salting out efficiencies reported in previous studies. Phase separations were observed after efflorescence for systems without LLPS. Our results provide a unique insight into the consecutive mixing processes of the inorganic−surfactant particles, which would help improve our fundamental knowledge of model development on radiative effect.